1. Field of the Invention
A Stockbridge vibration damper is a seismic device used for damping overhead power transmission lines. Stockbridge vibration dampers comprise of a pair of weights joined by a stranded steel cable (commonly known as a “messenger cable”) and a clamp attached to the cable at a location intermediate the weights for attachment to an overhead power transmission cable. The configuration of weights mounted on the ends of the messenger cable is designed to resonate at frequencies determined to be appropriate for the vibration occurring in the transmission line cable.
2. Description of the Prior Art
Stockbridge vibration dampers function in two ways:
(a) by exerting a force out of phase with the conductor vibration, and
(b) by dissipating the energy through flexing of the messenger cable. The flexing of the messenger cable causes inter-strand friction within the messenger cable which generates heat which is dissipated into the surrounding air.
A typical Stockbridge vibration damper employs a messenger cable comprised of two or more layers of helically wound strands of high tensile steel wire. For example, one common form of messenger cable comprises two layers of seven strands of helically wound high tensile steel wire. In recent years, greater interstrand friction has been obtained by using messenger cable with three strand layers comprising a total of 19 wires. As a result of the bending of the messenger wire caused by the movement of the weights those strands on the outside of the bend are subject to greater deflection than those on the inside of the bend so that with straight wires considerable sliding action and hence interstrand friction would occur.
As a consequence of the wires being helically stranded, the wire on the outside of the bend at one position becomes the wire on the inside of the bend within one pitch length and vice versa. That is, a strand which is on the inside of the bend at one point is transferred to the outside of the bend at another point. This has the effect of cancelling out any effective sliding between the wires caused by the bending of the cable.
It has been found, particularly within 19 strand messenger cable, that each strand layer acts as a combined member and experiences considerable sliding over the adjacent strand layer. In the construction of Stockbridge type vibration dampers, it is necessary to attach the damping weights securely to the ends of the messenger cable and this is conventionally done in a number of ways, such as:
(a) compressing the damping weights onto the messenger cable;
(b) moulding/casting the damping weights onto the ends of the messenger cable;
(c) by using a potting metal to secure the damping weights onto the ends of the messenger cable;
(d) by attaching a tapered collett onto each end of the messenger cable and then pressing the tapered collett into a complementary tapered hole in the damping weight; or
(e) passing all of the individual strands of the messenger cable through a hole in the damping weight and subsequently welding all of the strands onto the damping weight.
However, it has been found that in the region of the connection between the messenger cable and each of the damping weights the different layers of messenger cable become compressed or fused together such that sliding between the different layers is prevented. This in turn adversely affects the ability of the messenger cable to dissipate energy by means of friction between the layers.
It is therefore an object of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative.